Neurophagy, the phagocytosis of live neurons and synapses by glia, contributes to brain development and disease

It was previously thought that neurons were phagocytosed only when dead or dying. However, it is increasingly clear that viable synapses, dendrites, axons and whole neurons can be phagocytosed alive (defined here as neurophagy), and this may contribute to a wide range of developmental, physiological and pathological processes. Phagocytosis of live synapses, dendrites and axons by glia contributes to experience-dependent sculpting of neuronal networks during development, but excessive phagocytosis of synapses may contribute to pathology in Alzheimer's disease, schizophrenia and ageing. Neurons can expose phosphatidylserine or calreticulin, which act as eat me' signals provoking phagocytosis via microglial receptors, whereas sialylation of neuronal surfaces acts as a don't eat me' signal that inhibits phagocytosis and desialylation can provoke phagocytosis. Opsonins, such as complement components and apolipoproteins, are released during inflammation and enhance engulfment. Phagocytosis of neurons is seen in multiple human diseases, but it is as yet unclear whether inhibition of phagocytosis will be beneficial in treating neurological diseases. Here we review the signals regulating glial phagocytosis of live neurons and synapses, and the involvement of this phagocytosis in development and disease.